1. Field of the Invention
The present invention relates in general to a hydraulic stop member intended to be used on a hydraulic shock-absorber, particularly on a twin-tube hydraulic shock-absorber for a vehicle suspension. More specifically, the present invention relates to a so-called hydraulic compression stop member, i.e. a stop member that acts during the compression stroke of the shock-absorber.
2. Description of the Related Art
A twin-tube hydraulic shock-absorber for a vehicle suspension typically comprises an outer cylindrical tube, an inner cylindrical tube which is arranged coaxial to the outer cylindrical tube and defines with the latter an annular chamber filled in an upper portion thereof with gas, a rod which extends coaxially to the inner and outer cylindrical tubes and projects partially therefrom, and a piston which is slidably mounted in the inner cylindrical tube and is attached to the bottom end of the rod. The piston separates the inner volume of the inner cylindrical tube into an extension chamber and a compression chamber, which both contain a damping fluid (typically oil). The piston is provided with a first pair of one-way valves, namely a compensation valve which during the compression stroke of the shock-absorber controls the flow of the damping fluid from the compression chamber to the extension chamber and a rebound valve which during the extension stroke of the shock-absorber controls the flow of the damping fluid from the extension chamber to the compression chamber. A base valve assembly is mounted on the bottom of the shock-absorber and comprises a second pair of one-way valves, namely a compression valve which during the compression stroke controls the flow of the damping fluid from the compression chamber to the annular chamber and an intake valve which during the extension stroke controls the flow of the damping fluid from the annular chamber to the compression chamber. Typically, a hydraulic shock-absorber for a vehicle suspension is also provided with a first stop member, which works during the extension stroke of the shock-absorber, and a second stop member, which works during the compression stroke of the shock-absorber.
Italian patent application No. 102015000008777 in the Applicant's name discloses a hydraulic compression stop member for a hydraulic shock-absorber, particularly for a twin-tube hydraulic shock-absorber for a vehicle suspension, comprising a cup-shaped body, which is mounted in the compression chamber of the shock-absorber, coaxially thereto, and a piston, which is attached to an end of the rod of the shock-absorber, coaxially thereto, so as to slide within the cup-shaped body when the shock-absorber is close to the end-of-travel position of the compression stroke. The cup-shaped body comprises a side wall separate from the inner cylindrical tube of the shock-absorber and a bottom wall, the side wall and the bottom wall defining, along with the piston, a working chamber where a damping fluid of the shock-absorber is compressed by the piston as the latter slides in the working chamber towards the bottom wall of the cup-shaped body. Axial channels are provided on the inner surface of the side wall of the cup-shaped body for allowing the damping fluid to flow axially out of the working chamber as the piston slides in the working chamber towards the bottom wall of the cup-shaped body. The axial channels extend parallel to a longitudinal axis of the cup-shaped body and have a cross-section whose area decreases continuously along this axis towards the bottom wall of the cup-shaped body. The piston of the hydraulic stop member comprises a cylindrical body, which is attached to the rod of the shock-absorber and has an outer diameter smaller than the inner diameter of the side wall of the cup-shaped body, a seal ring, which is axially slidably mounted around the cylindrical body and is arranged to seal against the inner surface of the side wall of the cup-shaped body, and first and second ring-shaped abutment elements, which are axially restrained onto the cylindrical body and axially limit, in either direction, the axial sliding movement of the seal ring on the cylindrical body. The seal ring, the first abutment element and the second abutment element cooperate together such that when the seal ring slides along the inner surface of the side wall of the cup-shaped body during the compression stroke of the shock-absorber the seal ring abuts against the first abutment element and there is no flow of oil from one side of the seal ring to the other one, whereas during the extension stroke of the shock-absorber the seal ring abuts against the second abutment element and the oil is allowed to flow from one side of the seal ring to the other, namely towards the working chamber of the cup-shaped body.
By virtue of the special configuration of the axial channels made on the inner surface of the side wall of the cup-shaped body, the area of the flow cross-section through which the oil contained in the working chamber can flow out of the cup-shaped body decreases continuously towards the bottom wall of the cup-shaped body, and therefore the damping force generated by the stop member on the rod of the shock-absorber increases continuously and progressively as the latter moves towards the end-of-travel position of the compression stroke.
According to this known solution, a plurality of passages are also provided in the bottom wall of the cup-shaped body for allowing the oil to flow out of the working chamber of the cup-shaped body to limit the maximum pressure in that chamber. This allows to prevent the pressure in the working chamber of the cup-shaped body from reaching excessive values that might jeopardize the structural integrity of the stop member. Alternatively, or in addition, to the passages in the bottom wall of the cup-shaped body, a suitably sized ring gap in the seal ring may perform the function of limiting the maximum pressure in the working chamber of the cup-shaped body.
The passages in the bottom wall of the cup-shaped body and/or the ring gap in the seal ring are very simple solutions, from a structural point of view, to the problem of avoiding an excessive increase in the pressure in the working chamber of the hydraulic stop member. However, these solutions do not allow to effectively limit the pressure in the working chamber of the hydraulic stop member. Both the passages in the bottom wall of the cup-shaped body and the ring gap in the seal ring define an oil path which is always open and extends in parallel to the axial channels on the inner surface of the side wall of the cup-shaped body. The pressure drop along this path increases according to a parabolic law as the oil flow rate increases, that is to say, as the speed of the rod of the shock-absorber increases. Therefore, if the speed of the rod is very high the pressure in the working chamber of the hydraulic stop member may reach too high values.